Why would NBC replace Jay Leno when he's so dominant?

HERE’S A fascinating story,” the longtime host of NBC’s Tonight Show With Jay Leno told his audience last week, mentioning a recent Iowa Supreme Court ruling “that a dentist could legally fire his female assistant because he found her too sexy, too attractive, and a threat to his marriage. She didn’t do anything. She’s just good-looking.” The payoff: “Hey, I’ll bet that’s what happened to me here at NBC!”

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The irony—and it’s a rich one—is that Leno has never been more dominant: the Nielsen ratings for the second week in July have him beating CBS’s David Letterman by a 43 percent margin in the all-important “viewers 18 to 49” category (on which advertising rates are set) and swamping ABC’s Jimmy Kimmel by a crushing 75 percent. The “total viewers” number is a similar story, with Leno delivering 3.3 million compared with Letterman’s 2.6 million and Kimmel’s 2 million. Not counting a two-year rough patch after Letterman was enthroned at CBS in 1993, that makes 17 seasons in which Leno has won his time period since he started hosting the iconic show on May 25, 1992.